1. Field of the Invention
The present invention relates to a light emitting apparatus incorporating a semiconductor light emitting device using a nitride semiconductor (e.g., gallium nitride compound semiconductor InxAlyGa1xe2x88x92xxe2x88x92yN, where 0xe2x89xa6x, 0xe2x89xa6y, and x+yxe2x89xa61) and a method for mounting the light emitting device. In particular, the present invention relates to a light emitting apparatus incorporating a nitride semiconductor light emitting device including a substrate which transmits light therethrough and a method for mounting the light emitting device.
2. Description of the Related Art
Typically, a nitride semiconductor light emitting device is produced by depositing a semiconductor multilayer which includes a number of nitride semiconductor layers on an insulative substrate. The semiconductor multilayer is then partially etched into a step like shape. Then a p-type electrode and an n-type electrode are formed on the semiconductor multilayer. The n-type electrode is formed on a lower step of the partially etched semiconductor multilayer. Thus, the height of the n-type electrode differs from that of the p-type electrode. Typically, the n-type electrode is formed at a height which is about 1 xcexcm lower than that of the p-type electrode. One way to emit the light from the nitride semiconductor light emitting device is to include a light transmissive substrate. Conventional mounting methods for electrically connecting such p-type and n-type electrodes formed at different heights to respective electrodes of respective lead frames will be described below.
For example, Japanese Laid-open Patent Publication No. 9-181394 discloses a method in which a nitride semiconductor light emitting device provided with a height difference is mounted on a heat sink that is also provided with a height difference so as to conform with the height difference between the p-type electrode and the n-type, electrode of the light emitting device. FIG. 8 is a schematic diagram illustrating the mounting method. Referring to FIG. 8, a p-type electrode 71 and an n-type electrode 72 are formed on one side of a nitride semiconductor light emitting device 70. A height difference section 700 is provided in a heat sink 73 so as to match with the height difference between the p-type electrode 71 and the n-type electrode 72. The n-type and p-type electrodes 72 and 71 are connected to lead electrodes 75 and 74, respectively, with a conductive material 77.
Japanese Laid-open Patent Publication No. 6-177434 discloses another conventional method for mounting a nitride semiconductor light emitting device, in which the height of an n-type electrode is adjusted to match with that of a p-type electrode. FIG. 9 is a schematic diagram illustrating the mounting method. A thick n-type electrode 80 and a thin p-type electrode 81 are formed on a nitride semiconductor light emitting device 79 so that the surface heights thereof match with each other. The n-type and p-type electrodes 80 and 81 are surrounded by an insulative protective film 82, and connected to lead electrodes 85 and 86 via conductive adhesive layers 83 and 84, respectively.
The first conventional method shown in FIG. 8 has the following problems. In this method, the nitride semiconductor light emitting device 70 needs to be mounted on the heat sink 73 while the p-type electrode 71 and the n-type electrode 72 of the light emitting device 70 are simultaneously placed on (and thus connected to) the lead electrodes 74 and 75 of the heat sink 73, respectively. Therefore, the method requires a very high die bonding accuracy. This causes difficulties during the manufacture of the light emitting device.
The second conventional method shown in FIG. 9 has the following problems. When this method is used with an ordinary light emitting diode or laser, the n-type electrode 80 needs to be as thick as about 1 xcexcm. Such a thick electrode complicates the production process. In stead of making the n-type electrode 80 thicker than the p-type electrode 81, the conductive adhesive layer 83 may be made thicker than the conductive adhesive layer 84 to similarly compensate for the height difference. In such a case, during a die-bonding process for the nitride semiconductor light emitting device, the conductive adhesive material may extend beyond the periphery of the electrode 80 or 81, which may short-circuit the electrodes 80 and 81 with each other. Thus, the production yield may be reduced considerably.
In either one of the conventional methods described above, since the semiconductor layer side is fixed to the heat sink with an adhesive (such as a silver paste and a solder), the semiconductor layer tends to be distorted thereby shortening the lifetime of the nitride semiconductor light emitting device.
Another problem with a conventional light emitting device is that the emission efficiency is decreased because the emitting light is absorbed and scattered by the electrodes formed on the semiconductor layer.
As described above, it has been difficult to provide a method for mounting a nitride semiconductor light emitting device, which improves the emission efficiency and hence the lifetime thereof.
According to one aspect of the present invention, there is provided a light emitting apparatus including: a light emitting device for emitting light; and a first lead frame and a second lead frame on which the light emitting device is mounted. In this apparatus, the light emitting device includes: a substrate which is light transmissive, the substrate defining an output surface; a semiconductor layer formed on the substrate which includes a light emitting layer made of a nitride semiconductor; a first electrode provided lower than a plane running parallel through the light emitting layer with respect to the substrate; and a second electrode provided higher than the plane running parallel through the light emitting layer with respect to the substrate; the first lead frame includes a first lead pad section to which the first electrode is connected; the second lead frame includes a second lead pad section to which the second electrode is connected, at least one of the first and second lead frames includes a die pad section on which the substrate is mounted; and the die pad section does not substantially cover the first output surface.
In one embodiment of the present invention, the light emitting device includes a reflective layer which reflects the light generated in the light emitting layer toward the substrate.
In another embodiment of the present invention, the reflective layer is formed between the light emitting layer and the second electrode.
In still another embodiment of the present invention, the reflective layer is formed to surround the light emitting device.
In still another embodiment of the present invention, the semiconductor layer includes at least a pair of cladding layers, the light emitting layer being made of a gallium nitride type compound semiconductor and being formed between the cladding layers; the reflective layer is a multilayer film including at least two different types of layers which are alternately deposited on one another; and the at least two different types of layers include a first layer made of a nitride semiconductor and having a refractive index of n, and a thickness of xcex/(4xc2x7n1) (where xcex denotes an emission wavelength of the light emitting device), and a second layer made of a nitride semiconductor with a refractive index of n2 and having a thickness of xcex/4xc2x7n2.
In still another embodiment of the present invention, the semiconductor layer includes at least a pair of cladding layers, the light emitting layer being made of a gallium nitride type compound semiconductor and being formed between the cladding layers; the second electrode has a property of transmitting light with a specific wavelength: and the reflective layer is an insulative multilayer film which is formed on the second electrode and reflects the light with the specific wavelength.
In still another embodiment of the present invention, the semiconductor layer includes at least a pair of cladding layers, the light emitting layer being made of a gallium nitride type compound semiconductor and being formed between the cladding layers; the second electrode has a property of transmitting light with a specific wavelength; and the reflective layer includes: a first insulative layer formed on the second electrode, the first insulative layer reflecting light with a specific wavelength; and a second layer formed on the first layer, the second layer reflecting the light with the specific wavelength at a high reflectivity.
According to another aspect of the present invention, there is provided a method for mounting a light emitting device includes the steps of: forming a semiconductor layer on a substrate which is light transmissive, the semiconductor layer including a light emitting layer; forming a first electrode lower than a plane running parallel through the light emitting layer with respect to the substrate and a second electrode higher than the plane running parallel through the light emitting layer with respect to the substrate; placing first and second lead frames in a predetermined position with respect to a light emitting device, the first lead frame including a first lead pad section to which the first electrode is connected, the second lead frame including a second lead pad section to which the second electrode is connected, and at least one of the first and second lead frames including a die pad section; mounting the substrate onto the die pad section of the at least one of the first and second lead frames; and electrically connecting the first and second electrodes to the first and second lead pad sections, respectively, wherein the first and second lead pad sections are positioned farther away from the substrate than the first and second electrodes, respectively.
The present invention provides a mounting method in which at least one edge or the periphery of an insulative substrate in a gallium nitride type compound semiconductor light emitting device is fixed to a die pad section of a lead frame, thus allowing the light from the insulative substrate to be emitted efficiently therethrough. This invention also eliminates a short circuit between a p-type electrode and an n-type electrode which are formed on the nitride type semiconductor light emitting device by connecting the electrodes to respective lead pad sections of the lead frames by lead wires. Furthermore, an insulative adhesive is used to fix the substrate of the light emitting device to the die pad sections, thereby preventing an electric short circuit between the lead frames and the light emitting device. Thus, the present invention improves the production yield of a gallium nitride type compound semiconductor light emitting device. Preferably, the insulative adhesive is an epoxy resin, a polyester resin, or a cyanoacrylic resin, or the like, which is colorless and transparent and has a 90 percent or more of light transmissivity for a short wavelength range which includes the emission wavelength of a gallium nitride type compound semiconductor light emitting device. Furthermore, because the substrate side of the light emitting device is fixed to the die pad sections, and distortion of the light emitting device caused by the adhesive element is decreased. Therefore, the operating lifetime of the gallium nitride type semiconductor light emitting device can be prolonged.
The first gallium nitride type semiconductor light emitting device according to the present invention has at least one pair of cladding layers and a light emitting layer on an insulative substrate. The device further includes a multilayer film between one of the electrodes (which is farther away from the substrate) and the light emitting layer. The multilayer film includes at least two types of nitride semiconductor layers. For example; when two different types of nitride semiconductor layers are included in the multilayer film, the respective thicknesses thereof are xcex/4n1 and xcex/4n2 (where xcex denotes the emission wavelength of the device, and n1 and n2 denote the respective refractive indices for the emission wavelength xcex). This prevents the light generated in a light emitting layer and directed away from the substrate from being absorbed and scattered by the electrode. Therefore, the light is more efficiently reflected by the multilayer film toward the light transmissive substrate. Thus, a gallium nitride type compound semiconductor light emitting device which has an excellent emission efficiency can be obtained. Preferably, each of the two types of nitride semiconductor layers is doped with a p-type impurity or an n-type impurity in order to facilitate the passage of an externally injected current. The multilayer film preferably includes about 10 to about 100 pairs of such two different types of semiconductor layers. Below about 10 pairs, a sufficient refractive index may not be obtained. Over about 100 pairs, the series resistance of the multilayer film, and hence the drive voltage, of the light emitting device increases. Each of the two types of nitride semiconductor layers may be an InxAlyGa1xe2x88x92xxe2x88x92yN layer (0xe2x89xa6x, 0xe2x89xa6y, and x+yxe2x89xa61), preferably, the two layers are a GaN layer and an AlxGa1xe2x88x92xN layer (0 less than xxe2x89xa60.3), respectively. In the latter, the Al molar fraction is more than 0 and about 0.3 or less. If the Al molar fraction in the AlxGa1xe2x88x92xN layer (0 less than xxe2x89xa60.3) is more than about 0.3, the resistance of the layer may be undesirably high, resulting in an undesirable conductivity.
The second gallium nitride type compound semiconductor light emitting device according to the present invention has at least one pair of cladding layers and a light emitting layer on an insulative substrate. The device further includes a thin film electrode on the side opposite to the insulative substrate with respect to the light emitting layer, an insulative multilayer film, and pad electrodes. These elements are deposited in this order. The insulative multilayer film has at least two different types of insulative layers, in which the respective thicknesses thereof are xcex/4n1 and xcex/4n2 (where xcex denotes the emission wavelength of the device, and n1 and n2 denote the respective refractive indices for the emission wavelength xcex). Therefore, the light generated in a light emitting layer and directed away from the substrate is prevented from being absorbed and scattered by the electrode. Since the insulative multilayer film is formed to surround the light emitting device except for the substrate, the light is more efficiently reflected by the multilayer film toward the light transmissive substrate. Thus, a gallium nitride type compound semiconductor light emitting device which has an excellent emission efficiency can be obtained. Preferred materials which may be used for one of the two types of insulative layers that has a higher refractive index include A12O3, CeF3, CeO2, HfO2, MgO, Nd2O3, NdF3, PbO, Pr6O11, Sc2O3, TiO2, TiO, Y2O3, ZrO2, etc. Preferred materials for the other insulative layer with a lower refractive index include CaF3, MgF2, LaF3, LiF, MgF2, Na3AlF6, NaF, SiO2, Si2O3, etc.
The third gallium nitride type compound semiconductor light emitting device according to the present invention has at least one pair of cladding layers and a light emitting layer on an insulative substrate. The device further includes a thin film electrode on the side opposite to the insulative substrate with respect to the light emitting layer. The device still further includes at least one type of an insulative layer and a metal layer having a high light reflectivity which are deposited to surround the light emitting device except for the substrate side, thereby the light is more efficiently reflected by the multilayer film toward the light transmissive substrate. Thus, a gallium nitride type compound semiconductor light emitting device which has an excellent emission efficiency can be obtained. For the above-mentioned metal, aluminum(Al) or silver(Ag) is preferred to be used since they have a high light reflectivity in a visible wavelength range.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.